Neural circuits are organized to elicit appropriate behavioral responses to environmental stimuli. Advancement in technologies allowing the visualization and manipulation of neural activity has made it possible to establish a causal relationship between circuit function and behavior. This proposal attempts to expand the repertoire of existing technologies by exploiting a unique feature of the NFAT molecule - calcium/calcineurin-dependent nuclear localization. In the novel activity reporter system named CaLexA, a truncated version of NFAT containing the necessary nuclear localization signal is fused to LexA to make the synthetic transcription factor LexA- NFAT. The resulting activity reporter system will detect not only persistent neural activity but phasic activity as well with hih sensitivity in freely moving, behaving animals. We have identified several state-of-the-art molecular technologies that will increase the signal- to-noise ratio of the CaLexA activity reporter system as well as add the ability to discriminate between phasic and sustained neural activity. In Aim 1, we will engineer new LexA-NFAT constructs to suppress unwanted background and enhance behaviorally relevant signal. In Aim 2, we will design new LexA-NFAT constructs that will allow the detection of transient calcium activity. In Aim 3, we will use optogenetics to generate precisely controlled patterns of neural activity so that calcium imaging and the CaLexA reporter system can be compared for their sensitivity and dynamic range to gain a full characterization of all the LexA-NFAT variants. The creation and characterization of these new CaLexA activity reporter systems will make available to the Drosophila neurobiology community a new set of tools for the study of neural circuits and behavior, and molecular designs that can be readily modified for other genetic model organisms.